<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const arr = ["a", "b", "c", "a"];

      console.log(arr.indexOf("c"));
      console.log(arr.indexOf("e"));
      console.log("indexOf(a)", arr.indexOf("a")); //0
      console.log("lastIndexOf(a)", arr.lastIndexOf("a")); //3
      console.log(arr.includes("c")); //true
      console.log(arr.includes("e")); //false
    </script>
  </body>
</html>
